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Alcools: mixture ethanol/water

Experiment

In two graduated cyclinders we have exactly 50 ml of water respectively 50 mL of ethanol. Here is what happens when they get mixed:

(Click on the image!)

The volume of the mixture is only worth $96\;mL$! (actually $96.26\; mL$). There was a volume contraction ! The final mass however is still equal to the sum of the two separate masses! (Law of Lavoisier)

Interpretation

Imagine a bag of marbles and a bag of the same volume filled with sand. Mix both. Can you complete the two bags?

No, of course, the sand will occupy the space between the marbles and the volume will be smaller than the full volume of the two bags. It is the same for molecules of ethanol("beads") and water molecules(the "sand")